The present invention relates to the configuration of an optical waveguide multiplexer for use in optical fiber amplifiers and the like.
In an optical communication system for long-distance transmission, feeble optical signals attenuated on the transmission path have to be amplified in optical repeaters and receivers arranged on the path. Recently, as means for such amplification, optical fiber amplifiers for directly amplifying optical signals without having to convert them into electric signals are coming into practical use.
A typical configuration of such an optical amplifier is illustrated in FIG. 1. An optical fiber amplifier 11 comprises optical isolators 16 and 18, optical branching filters 13 and 19, an optical waveguide multiplexer 15, pumping light source 26, a rare earth ion-doped optical fiber 17 for performing optical amplification, and monitor photodiodes (monitor PD's) 22 and 15. Part of a signal light P.sub.1 of wavelength .lambda..sub.1, attenuated on the transmission path, is branched into a light P.sub.2 by the optical branching filter 13 as monitor light, and entered into the monitor PD 22. The remaining light is transmitted to the optical waveguide multiplexer 15, and multiplexed with a pumping light P.sub.3 of wavelength .lambda..sub.2, which is supplied from the pumping light source 26, to output a light P.sub.4. The light P.sub.4 further passes the isolator 16 and is transmitted over the rare earth ion-doped optical fiber 17 for the amplifying purpose. The original signal light P.sub.1 is optically amplified into a light P.sub.5 to pass the isolator 18. At this time, part of the light P.sub.5 is branched by the optical branching filter 19 into a light P.sub.6 as monitor light to be entered into the monitor PD 25, and the remaining light P.sub.7 is supplied from the optical fiber amplifier 11. Here, the wavelength .lambda..sub.1 of the normally transmitted signal light differs from the wavelength .lambda..sub.2 of the pumping light. For instance, against a signal light of .lambda..sub.1 =1.55 .mu.m, at which the transmission loss of the optical fiber is minimized, a pumping light of .lambda..sub.2 =1.48 .mu.m or 0.98 .mu.m, at which the rare earth ion-doped optical fiber can achieve a large amplification gain, is used.
Any optical fiber amplifier according to the prior art has a configuration in which the optical branching filter for branching part of the entered signal light to a monitor PD and the optical waveguide multiplexer for multiplexing the pumping light from an excited light source are separately provided. Regarding the case in which the optical branching filter and the optical waveguide multiplexer are to be separately provided, the internal configuration of each unit will be described below.
FIG. 2 specifically illustrates the configuration of the first optical branching filter 13 as an example of optical branching filter used in this prior art optical fiber amplifier. Incidentally, the configuration of the second optical branching filter 19 is substantially the same as that of the first optical branching filter 13.
The first optical branching filter 13 is provided with three input/output (I/O) terminals 3.sub.11 to 3.sub.13 including the first I/O terminal 3.sub.11 ; the second I/O terminal 3.sub.12 arranged in a direction normal to the travelling direction of an optical signal 32 coming incident via the first I/O terminal 3.sub.11 ; and the third I/O terminal 3.sub.13 arranged in the travelling direction of the optical signal 32 coming incident via the first I/O terminal 3.sub.11. Within the first optical branching filter 13 is arranged a glass plate 33 inclined at a prescribed angle to the travelling direction of the optical signal 32. A branching film 34 is formed on the face where the optical signal 32 comes incident, and an anti-reflection film 35, on the opposite face. In the first optical branching filter 13, when the optical signal 32 of wavelength .lambda..sub.1, for instance, comes incident via the I/O terminal 3.sub.11, it is partly branched by the branching film 34, and the branched part is radiated via the second I/O terminal 3.sub.12. The rest of the optical signal penetrates the glass plate 33, and is radiated via the third I/O terminal 3.sub.13.
FIG. 3 specifically illustrates the configuration of the prior art waveguide multiplexer used in the optical fiber amplifier shown in FIG. 2. The waveguide multiplexer 15 is provided with three I/O terminals 4.sub.11 to 4.sub.13 including the first I/O terminal 4.sub.11 ; the second I/O terminal 4.sub.12 arranged in the travelling direction of an optical signal 27 coming incident via the first I/O terminal 4.sub.11 ; and the third I/O terminal 4.sub.13 arranged in a direction normal to the travelling direction of the optical signal 27. Within this waveguide multiplexer 15 is arranged a glass plate 43 inclined at a prescribed angle to the travelling direction of the optical signal 27. An anti-reflection film 44 is formed on the face where the optical signal 27 comes incident, and an optical multiplexing film 45, on the opposite face.
In the optical waveguide multiplexer 15, the optical signal 27 of wavelength .lambda..sub.1 comes incident via the first I/O terminal 4.sub.11, and a pumping light 14 of .lambda..sub.2 comes incident via the third I/O terminal 4.sub.13. The optical signal 27 penetrates the anti-reflection film 44, the glass plate 43 and the optical multiplexing film 45 successively, and is led to the I/O terminal 4.sub.12. On the other hand, the pumping light 14 is reflected by the optical multiplexing film 45, and similarly led to the I/O terminal 4.sub.12.
In this manner, in the prior art optical fiber amplifier, the first optical branching filter 13 and the waveguide multiplexer 15 are separately arranged as physically different parts on the input side of the rare earth ion-doped fiber 17. As a result, not only is the number of parts increased but also there is the need to adjust the relative positions of the first optical branching filter 13 and the waveguide multiplexer 15. This conventional configuration, in which the optical branching filter and the optical waveguide multiplexer are separately provided, has the disadvantage of an increased overall size due to the large number of parts constituting the optical fiber amplifier. The large number of constituent parts, including altogether six optical I/O terminals being which are required, further creates the problem of a greater manufacturing cost.
There is another disadvantage of a significant loss of signal light because the signal light is collimated to enable it to penetrate the optical branching film in the optical branching filter, and the signal light is again condensed into a parallel beam to let it penetrate the optical multiplexing film. In view of these problems, there has been proposed a multiplexer-branching filter in which the two constituent units are integrated.
FIG. 4 specifically illustrates the configuration of this proposed multiplexer-branching filter. This multiplexer-branching filter 51 is provided with a total of four I/O terminals 5.sub.21 to 5.sub.24 including the first I/O terminal 5.sub.21 ; the second I/O terminal 5.sub.22 arranged in a direction normal to the travelling direction of an optical signal 12 of wavelength .lambda..sub.1, coming incident via the first I/O terminal 5.sub.21 ; the third I/O terminal 5.sub.23 arranged in the travelling direction of the optical signal 12; and the fourth I/O terminal 5.sub.24 arranged in a position opposite to the second I/O terminal 5.sub.22.
On the optical path where the first I/O terminal 5.sub.21 and the third I/O terminal 5.sub.23 are coupled in the optical multiplexer-branching filter 51, a first glass plate 53 and a fourth glass plate 54 are arranged at a prescribed interval, both forming the same angle of inclination. A branching film 55 is formed on the incidence face of the first glass plate 53, and an anti-reflection film 56, on the opposite face. An anti-reflection film 57 is formed on the incidence face of the second glass plate 54, and an optical multiplexing film 58, on the opposite face.
In the optical multiplexer-branching filter 51, the optical signal 21 reflected by the branching film 55 of the first glass plate 53 is supplied from the second I/O terminal 5.sub.22 as monitor light. The monitor light is further brought to incidence on the first monitor PD 22 shown in FIG. 2. On the other hand, the optical signal having penetrated the branching film 55, the first glass plate 53 and the anti-reflection film 56 further penetrates the other anti-relfection film 57, the second glass plate 54 and the optical multiplexing film 58. As the pumping light 14 of wavelength .lambda..sub.2, coming incident via the fourth I/O terminal 5.sub.24, is reflected by this optical multiplexing film 58, the lights of the two wavelengths .lambda..sub.1 and .lambda..sub.2 are multiplexed and led to the third I/O terminal 5.sub.23.
As the branching film 55 and the optical multiplexing film 58 are formed on the separate glass plates 53 and 54 in the optical multiplexer-branching filter 51 illustrated in FIG. 4, they cannot be integrated into one and the same constituent part. Therefore, the number of parts is still great, and there is the problem that the size of the multiplexer-branching filter 51 as such is larger than a branching filter or a waveguide multiplexer according to the prior art. There is another problem of the structural complexity of the multiplexer-branching filter 51.
Whereas an optical multiplexer-branching filter of another configuration is disclosed in the Gazette of Patent Disclosure No. 1991-225304, this is an optical element for separating a mixture of lights of two wave-lengths .lambda..sub.1 and .lambda..sub.2 into its constituent lights, different from a multiplexer-branching filter for performing both branching and multiplexing, and accordingly has no direct relevance to the present invention.